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aes(n) 1.1 "Advanced Encryption Standard (AES)"

Name

aes - Implementation of the AES block cipher

Description

This is an implementation in Tcl of the Advanced Encryption Standard (AES) as published by the U.S. National Institute of Standards and Technology [1]. AES is a 128-bit block cipher with a variable key size of 128, 192 or 256 bits. This implementation supports ECB and CBC modes.

COMMANDS

::aes::aes ?-mode [ecb|cbc]? ?-dir [encrypt|decrypt]? -key keydata ?-iv vector? ?-hex? ?-out channel? ?-chunksize size? [ -in channel | ?--? data ]

Perform the aes algorithm on either the data provided by the argument or on the data read from the -in channel. If an -out channel is given then the result will be written to this channel.

The -key option must be given. This parameter takes a binary string of either 16, 24 or 32 bytes in length and is used to generate the key schedule.

The -mode and -dir options are optional and default to cbc mode and encrypt respectively. The initialization vector -iv takes a 16 byte binary argument which defaults to all zeros. See MODES OF OPERATION for more about available modes and their uses.

AES is a 128-bit block cipher. This means that the data must be provided in units that are a multiple of 16 bytes.

PROGRAMMING INTERFACE

Internal state is maintained in an opaque structure that is returned from the Init function. In ECB mode the state is not affected by the input but for CBC mode some input dependent state is maintained and may be reset by calling the Reset function with a new initialization vector value.

::aes::Init mode keydata iv

Construct a new AES key schedule using the specified key data and the given initialization vector. The initialization vector is not used with ECB mode but is important for CBC mode. See MODES OF OPERATION for details about cipher modes.

::aes::Encrypt Key data

Use a prepared key acquired by calling Init to encrypt the provided data. The data argument should be a binary array that is a multiple of the AES block size of 16 bytes. The result is a binary array the same size as the input of encrypted data.

::aes::Decrypt Key data

Decipher data using the key. Note that the same key may be used to encrypt and decrypt data provided that the initialization vector is reset appropriately for CBC mode.

::aes::Reset Key iv

Reset the initialization vector. This permits the programmer to re-use a key and avoid the cost of re-generating the key schedule where the same key data is being used multiple times.

::aes::Final Key

This should be called to clean up resources associated with Key. Once this function has been called the key may not be used again.

MODES OF OPERATION

Electronic Code Book (ECB)

ECB is the basic mode of all block ciphers. Each block is encrypted independently and so identical plain text will produce identical output when encrypted with the same key. Any encryption errors will only affect a single block however this is vulnerable to known plaintext attacks.

Cipher Block Chaining (CBC)

CBC mode uses the output of the last block encryption to affect the current block. An initialization vector of the same size as the cipher block size is used to handle the first block. The initialization vector should be chosen randomly and transmitted as the first block of the output. Errors in encryption affect the current block and the next block after which the cipher will correct itself. CBC is the most commonly used mode in software encryption. This is the default mode of operation for this module.

EXAMPLES

% set nil_block [string repeat \\0 16]
% aes::aes -hex -mode cbc -dir encrypt -key $nil_block $nil_block
66e94bd4ef8a2c3b884cfa59ca342b2e
set Key [aes::Init cbc $sixteen_bytes_key_data $sixteen_byte_iv]
append ciphertext [aes::Encrypt $Key $plaintext]
append ciphertext [aes::Encrypt $Key $additional_plaintext]
aes::Final $Key

REFERENCES

  1. "Advanced Encryption Standard", Federal Information Processing Standards Publication 197, 2001 (http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf)

AUTHORS

Thorsten Schloermann, Pat Thoyts

BUGS, IDEAS, FEEDBACK

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category aes of the Tcllib SF Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

Category

Hashes, checksums, and encryption

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